The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
Optical coherent transmission attracts more and more interests for its higher performance over direct detection. In intradyne coherent optical transmission, the local laser at the receiver side may not be able to track the frequency of the laser at the transmitter side. So a carrier frequency offset occurs between both sides. This frequency offset results in a middle-frequent envelop upon the received signal. This effect should be compensated using digital signal processing (DSP) algorithm, in the receiver.
In the existing art (see for example Prior Art Documents [1], [2], [3], [4] and [5]), the solution to compensate the carrier frequency offset is at first to estimate the frequency offset and convert the frequency offset to phase offset of each sampled signal, then rotate the signal with the estimated phase in the contrary direction. According to Prior Art Document [1] the frequency offset is compensated through inverse rotation of the signal. The inverse rotation is implemented in the time domain. For each sample signal, according to the latter document the inverse rotation needs 4 real number multiplications, 3 addition, 1 modulo operation, 1 shifting operation to be performed.
However, optical communication has large capacity, for example 40 Gbit/s and above. Even if quadrature phase shift keying (QPSK) plus polarization multiplexing is employed, the symbol rate is still as high as 10 Gsymbol/s. With such high speed, even a simple multiplier turns to be a challenge for DSP. Prior Art Document [5] discloses a pre-decision phase recovery for optical QPSK coherent receivers. The proposed method only uses adder, subtractor, look up table, and some logical circuit, but no multiplier. Compared with conventional 4th power method developed by Viterbi and Viterbi this method according to Prior Art Document [5] requires reduced calculation time. However, in practice it seems not to be easy to implement a simple receiver using such method.
In addition to the carrier frequency offset, optical dispersion of the fiber causes the transmitting signal being distorted. The receiver must compensate the distortion to improve the signal quality to be correctly detected. Therefore in optical signal reception systems equalization of optical reception signals is performed. Such equalization can for example be chromatic dispersion compensation in order to compensate chromatic dispersion and/or polarization mode dispersion compensation applied to equalize polarization mode dispersion. In the existing art like Prior Art Document [6], compensation of frequency offset and one ore more types of equalization including for example compensation of chromatic dispersion are performed independently. According to Prior Art Document [6] in order to perform optical distortion compensation filtering can be performed in the time domain or using fast convolution techniques in the frequency domain in order to minimize the overall DSP complexity. I.e., even if filtering in frequency domain requires performing convolution like Fast Fourier Transform (FFT) and additionally inverse convolution like Inverse Fast Fourier Transform (IFFT), depending on the filtering method such filtering in frequency domain can require less calculation effort than performing a required filtering in time domain.
Performing several complicated compensations involves a complex receiver structure.